The present invention is directed generally to progressive multifocal ophthalmic lenses and, in particular, to a new configuration for the refractive surface of a progressive multifocal ophthalmic lens.
Progressive multifocal ophthalmic lenses have been developed to compensate for the decreased ability of the eye to control the crystalline lens in an aged person. A number of types of such progressive multifocal ophthalmic lenses have been disclosed, including those disclosed in U.S. Pat. No. 3,687,528, U.S. Pat. No. 3,910,691, U.S. Pat. No. 4,056,311, Canadian Patent No. 1,152,369, U.S. Pat. No. 4,240,719, U.S. Pat. No. 4,315,673, U.S. Pat. No. 4,253,747 and British Patent No. 2,092,772A. The present inventors are named as inventors in U.S. patent application Ser. No. 180,765, now abandoned and U.S. patent application Ser. No. 327,288, which also disclose different types of progressive multifocal lenses.
The basic construction of progressive multifocal ophthalmic lenses disclosed in the above patents or patent applications is common, as described below.
A progressive multifocal ophthalmic lens generally includes a segment for viewing distant objects and a segment for viewing nearby objects at the upper and the lower portions of the lens, respectively, and a third segment for viewing intermediate objects between these two segments. These three segments are called "the far vision viewing zone" (hereinafter referred to as "the far zone"), "the rear vision viewing zone" (hereinafter referred to as "the near zone") and "the intermediate vision viewing zone" (hereinafter referred to as "the intermediate zone"), respectively, and are divided into left and right parts by a principal meridian curve which extends vertically. In at least the intermediate zone, the surface power varies progressively. The demarcations of these segments are made to be smooth so that the demarcations are not perceived by the wearer of the lens. These three segments are usually provided on the convex surface of the two refractive surfaces, convex and concave, constituting the lens. The other surface of the lens is then provided with a spherical or toric surface designed specifically to correct farsightedness, nearsightedness, astigmatism and the like of the wearer.
The basic construction of the conventional progressive multifocal ophthalmic lens is described in more detail with reference to the drawings.
FIG. 1 is a plan view of the refractive surface of a lens body 10 for a progressive multifocal lens, showing the arrangement of the different segments described above. In FIG. 1, the far zone is designated at 1, the intermediate zone is designated at 2, and the near zone is designated at 3, and M is the principal meridian curve.
FIG. 2 illustrates the variation of the surface power along the principal meridian curve M. The surface power is expressed by: EQU surface power=C.times.(N-1)
where C is the curvature in the units of m (meters), and N is the refractive index of the lens material, and the units of the surface power is diopter (hereinafter referred to as D).
As shown by FIGS. 1 and 2, the surface power along the principal meridian curve above the point A, that is, in the far zone, is D1, and that below the point B, that is, in the near zone, is D2. Between points A and B, the surface power progressively increases from D1 to D2. The difference between D1 and D2 (D2-D1) is referred to as the additional power, which additional power is usually between 0.5 to 3.5D. In FIG. 2, the surface powers in the far zone and the near zone, respectively, are constant as an example. However, as will be described later, there is another example in which the surface power in at least one of the far and the near zones progressively varies. In such a case, D1 and D2 are not defined, and consequently, the additional power of the lens cannot be evaluated. Accordingly, in order to determine the additional power of the lens of this kind, the reference surface power is defined in each zone. Hereinafter, D1 and D2 represent the far-zone reference focal power and the near-zone reference focal power, respectively.
In FIG. 1, the length L between points A and B is referred to as the length of the intermediate zone or the length of the progressive portion.
As described above, in the progressive multifocal ophthalmic lens, since a plurality of separate segments of different focal powers are combined into one smooth curved surface, at least the intermediate zone is inevitably aspherical. As a result, astigmatism appears in the peripheral portion of the lens. In addition, since the magnification of images seen through each portion of the refractive surface is different, distortion of the images may occur. These defects are illustrated in FIGS. 3 and 4.
FIG. 3 is a contour of the astigmatism for explaining the distribution of the astigmatism of the lens of FIG. 1. Herein, the astigmatism is obtained by coverting the difference in the principal curvatures on the refractive surface into the differences in the surface power. That is, since the refractive surface of the progressive multifocal ophthalmic lens is aspherical, the curvature at a certain point (or a minute plane) is different depending upon the direction chosen. The maximum and minimum of the curvatures in the various directions at a certain point are called the principal curvatures. Given that the principal curvatures expressed in units of m.sup.-1 are C1, C2, astigmatism is obtained by the formula: EQU astigmatism=.vertline.C1-C2.vertline..times.(N-1)
and the units are D. Astigmatism is perceived by the wearer of the lens as the blurring of the images, and astigmatism exceeding 0.5D usually causes the wearer to become dizzy. In FIG. 3, the denser the hatched lines are, the larger the astigmatism is, and consequently, the more severe the blurring of the images is.
The principal meridian curve usually forms an umbilical curve. The umbilical curve is a series of points where the principal curvatures are equal, that is, of minute spherical surfaces, along which the astigmatism is essentially 0. Even if the principal meridian curve does not form an umbilical curve, the astigmatism along the principal meridian curve is made to be smallest.
FIG. 4 illustrates distortion of the images of the square grid when viewed through the progressive multifocal ophthalmic lens of FIG. 1. The difference in the magnification in each portion of the refractive surface causes distortion of the images of the square grid, in which the vertical lines laterally expand into the downward direction with respect to a center line 41 corresponding to the principal meridian curve of the lens, and the horizontal lines skew downward in the peripheral portions. Such a skew distortion of the images is not only perceived as a distortion of the vision by the wearer, but also causes shaking of the images when the objects relatively move with respect to the line of vision of the wearer, such as when the wearer follows an object with his eyes or watches something while turning his head, resulting in the wearer becoming dizzy and nauseous.
In making spectacles having progressive multifocal lenses, the lens body 10 is cut into the internal eye shape. In the cutting process, it is required to define the fitting point and the insetting of lenses to adapt for the convergence.
The fitting point is the position on the refractive surface of the lens through which the line of vision of the wearer passes when he looks at the far vision in the natural position and is sometimes called "the eye point."Generally, the fitting point is defined on the principal meridian curve between point A and a point 2 to 3 mm above A. In FIG. 3, the fitting point F is defined on point A.
The convergence means that the line of vision moves more inside when looking at nearby objects than when looking at distant objects. Accordingly, when making spectacles, the lenses are required to be arranged so that the distance between the points B of both lenses is shorter than the distance between the points A of both lenses. In other words, the lenses are inset. In general, the lens as shown by FIG. 3 is designed so that the left and the right half are symmetrical with respect to the principal meridian curve M and is used for spectacles by rotating by an angle of about 10.degree.. For example, presuming that the lens of FIG. 3 is viewed from the side of the convex surface of the lens, the lens is used as a spectacle lens for the left eye under the condition that the horizontal line H (the line orthogonal to the principal meridian curve M) is rotated by about 10.degree. to the line H' (hereinafter referred to as the horizontal line when glazed H'). Accordingly, the configuration of the lens for spectacles after the cutting process is shown by 11. In FIG. 3, only the configuration of the lens for the left eye is described but not for the right eye. When preparing the spectacle lens for the right eye, the horizontal line H is rotated into the opposite direction to the horizontal line when glazed H' for the left eye.
There is another type of progressive multifocal ophthalmic lens, as shown by FIG. 5, in which the left and the right halves of the refractive surface thereof are asymmetrical. In the case of FIG. 5, the principal meridian curve M is inclined in the middle portion, and the horizontal line H of the lens need not be rotated for use in spectacles. The lens of FIG. 5 is designed for the left eye and 11 is the configuration of the lens after the cutting process. In designing this type of ophthalmic lens for the right eye, the principal meridian curve M between the points A and B is inclined in the opposite direction from the direction shown in FIG. 5.
The above-described progressive multifocal ophthalmic lenses are only a few examples of the conventional lenses, and there are many other lenses having the same basic construction and still giving the wearer different wearing sensations. The fact that there are so many kinds of progressive multifocal ophthalmic lenses demonstrates that the ideal design for a progressive multifocal ophthalmic lens is difficult to realize. In other words, in designing a progressive multifocal ophthalmic lens, there is a significant problem that if a characteristic is improved, another characteristic is adversely affected.
Among many characteristics of the progressive multifocal ophthalmic lens affecting each other, those which restrict the design of lenses most considerably are called "the dynamic vision" and "the static vision."
"The dynamic vision" is the vision in the case where the object moves relative to the line of vision, such as when viewing a moving object or watching something while turning one's head, and "the static vision" is the vision in the case where the line of vision and the object are both almost still. With respect to the design of progressive multifocal ophthalmic lenses, the dynamic vision is affected mainly by distortion of the images, and the smaller the distortion of images is, the better the dynamic vision is. On the other hand, the static vision is affected mainly by astigmatism, and the smaller the astigmatism on the total refractive surface of the lens or the larger the area of the region having the small astigmatism (for example, the region having the astigmatism is no more than 0.5D) is, the better the static vision is.
If the region having smaller astigmatism is designed to be larger in order to obtain good static vision, the magnification changes abruptly around the region, that is, in the lateral portions of the lens, and thus the distortion of images becomes severe, deteriorating the dynamic vision. In contrast, if the dynamic vision is improved, the area of the region with the small astigmatism in the far and near zones is reduced, and the static vision is affected.
Accordingly, the balance of the dynamic vision and the static vision is considered to be one of the most important factors in designing the progressive multifocal ophthalmic lens. It can be said that the difference in design of the progressive multifocal ophthalmic lens is ultimately the difference in degree of giving priority to either the dynamic or the static vision. In some lenses, as a result of thinking more highly of the improvement of the static vision than of the dynamic vision, the refractive surface in the far zone as a whole is made to be spherical, and a large spherical part is also provided in the center of the near zone. By this structure, while the static vision is improved, the distortion of images is very severe in the lateral portions of the intermediate and near zones, and the dynamic vision is deteriorated. In another lens, as a result of thinking more highly of the improvement of the dynamic vision than of the static vision, the far and near zones are both provided with aspherical surfaces in order to reduce the distortion of the images as a whole. Accordingly, the region with the small astigmatism becomes narrow, and the static vision is affected.
In spite of a difference in the priority of the dynamic and the static visions as mentioned above, in the conventional progressive multifocal ophthalmic lenses, designers have had one common basic conception. That is, they have pursued the design of the progressive multifocal lens which is available in various circumstances, i.e., which is used for various purposes. In general, the progressive multifocal ophthalmic lens has been designed for the presbyopic, and while the priorities of the far, intermediate and near zones are almost equal, the area of the near zone tends to be larger than that of the other zones.
With respect to a certain specific use, the conventional progressive multifocal lens for the various purposes is not necessarily most suitable, and it is often very inconvenient, for example, when engaging in sports (such as golf), shopping, driving a car and the like. The requirements in designing of progressive multifocal ophthalmic lens for use in such conditions as above may not suffice in the conventional progressive multifocal lens for the various purposes mentioned above. Some of the lenses with a small additional power (0.5 to 1.25D) do have appropriate characteristics (it is natural because if the additional power is small, the astigmatism can be essentially reduced). However, although moderate or large additional powers are required in the progressive multifocal ophthalmic lenses, the conventional lenses with such large additional powers do not have aptitude for use in the above circumstances.
Accordingly, improved multifocal ophthalmic lenses are desired.